PROJECT SUMMARY Breast and ovarian cancer are two of the most common cancers among women in the US. Approximately 15% to 20% of cases exhibit a family history of breast or ovarian cancer suggesting strong heritable components. Clinical germline genetic testing of cancer predisposition gene panels is now widely used in an effort to identify women at risk for these cancers. The identification of pathogenic mutations in established predisposition genes can result in improved risk management for ovarian or breast cancer, depending on the gene, for tested patients and their family members. For instance, MRI screening for early detection of breast cancer is recommended for all individuals with pathogenic mutations in high and moderate risk breast cancer genes, and carriers of BRCA1 and BRCA2 mutation carriers can benefit from prophylactic mastectomy and/or oophorectomy for reduction of cancer risk. Furthermore, ovarian cancer patients with germline or somatic BRCA1 and BRCA2 mutations may benefit from targeted therapy with platinum agents or PARP inhibitors. The efficacy of these clinical interventions show the promise of identifying pathogenic mutations in genes that predispose to these cancers. However, the utility of results from panel testing has been limited because the levels of risk for breast and ovarian cancer associated with mutations in several of the panel genes has not been clear. Importantly, recent studies have established that germline pathogenic mutations in RAD51C and RAD51D are associated with substantially increased risks of ovarian cancer, and moderate and high risks of triple negative breast cancer (TNBC), respectively, and RAD51D mutations are associated with moderate risks of breast cancer overall. While these genes have important roles in susceptibility to breast and ovarian cancer, much remains to be learned about how to use testing results for the benefit of those with germline pathogenic mutations or variants of uncertain significance or with somatic inactivation of the genes in tumors. In this precision medicine oriented project we propose to define the contribution of RAD51C and RAD51D mutations to ovarian and breast cancer. In Aim 1 we propose to define the risks of ovarian and breast cancer associated with inherited RAD51C and RAD51D mutations in high-risk families and the general population. In Aim 2, we propose to comprehensively characterize the functional properties and functional domains of RAD51C and RAD51D that influence cancer risks and response to therapeutic agents, leading to classification of the clinical relevance of many variants of uncertain significance (VUS). In Aim 3, we propose to evaluate the influence of targeted therapy on RAD51C and RAD51D deficient tumors in preclinical models. At the conclusion of the study we expect to have improved understanding of RAD51C and RAD51D-associated cancer susceptibility and the responsiveness of ovarian tumors deficient in RAD51C or RAD51D to specific therapeutic agents.